Phased array antenna having a wave speeding ground plane

ABSTRACT

A phased array antenna consisting of a plurality of radiating elements associated with a ground plane and including a reactive impedance for reducing the catastrophic effect produced by endfire grating lobes and surface waves. A series capacitance or shunt inductance is placed in the transmission path formed by the ground plane and free space in order to increase the phase velocity of any TM wave that might propagate along the ground plane so that it is at least equal to the phase velocity of a free space wave.

[5 6] References Cited [2]] App No zrsnsnhmtgwn, N.Y. UNITED STATESPATENTS 221 Filed 3,259,902 7/1966 3,277,488 10/1966 Hofiman HaultineCorporation Primary E.xamr'nerEli Lieberman Attorney-Kenneth P. RobinsonPeter W. Harman Sept. 8, 1969 Assignee VII-UV vmwwu l l.

[72] Inventor [45] Patented May 4,1971

ABSTRACT: A phased array antenna consisting of a plurality of radiatingelements associated with a ground plane and including a reactiveimpedance for reducing the catastrophic ef- 343/778, fect produced byend-fire grating lobes and surface waves. A 343/785, 343/846, 343/91 1series capacitance or shunt inductance is placed in the trans- Int.mission path formed by the ground plane and free space in 343/753, orderto increase the phase velocity of any TM wave that 853, 854, mightpropagate along the ground plane so that it is at least 909, 91 1 equalto the phase velocity of a free space wave [54] PHASED ARRAY ANTENNAHAVING A WAVE SPEEDING GROUND PLANE 13 Chins, 18 Drawing Figs. [52] US.[5 l] ...............H0lq 13/00 [50] Field of 770, 771, 785, 778, 739,846,847, 868,

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PHASED ARRAY ANTENNA HAVING A WAVE SPEEDING GROUND PLANE BACKGROUND OFTHE INVENTION A phmed array antenna consisting of a plurality ofradiating elements is capable of radiating a narrow beam ofelectromagnetic energy which is steerable in space. The beam is radiatedin the desired direction by adjusting the phase of the power coupled toeach radiating element in a manner that causes the contribution fromeach element to be in phase along a phase front which is perpendicularto the desired direction of radiation. However, in addition to being inphase in the main beam the contributions from each element may also bein phase at, at least, one other angle thereby tending to form aspurious beam referred to as a grating lobe.

The angular relationship between the main beam and the grating lobe isdependent on the spacing between the radiating elements, specified inwavelengths. For an array substantially larger than a wavelength, if thespacing between the elements is small enough, less than 0.5A where A isthe wavelength of the propagating energy, the grating lobe will notexist in real space. However, there are usually other design criteriawhich require that the spacing between the elements be greater than 0.5aand the existence of grating lobes is usually a factor which must beconsidered in the design of phased array antennas.

While grating lobes are generally undesirable since they reduce thepower radiated in the main beam and can produce spurious responses,these disadvantagesare of a minor nature as compared to the catastrophiceffect which grating lobes can produce in phased array antennas whichconsist of slots or holes in a metal ground plane or otherwise consistof radiating elements asociated with a ground plane. Although this typeof array is very desirable from a mechanical and environmentalstandpoint, it has a basic defect in electrical performance. When themain beam is steered sufficiently far from broadside, the grating lobeappears in the end-fire direction, the direction'of radiation coplanarwith the ground plane, producing a drastic reduction in radiated power.Substantially all the power is reflected from the array and there is nosignificant power radiated.

There are many instances when the ground plane departs from a simpleflush configuration such as when the ground plane is covered withdielectric material in order to provide environmental protection for theradiating elements. The departure from a flush configuration usuallyresults in the propagation of surface waves along the ground plane. Atcertain scan angles surface wave propagation can cause an almost totalreflection of radiated power.

The scan angle at which this complete reflection due to sur-, face wavepropagation occurs is often less than the scan angle in whichpropagation of the grating lobes in the end-fire direction causes almostcomplete reflection of the energy. The scan angle is the angle betweenthe direction of the main beam and the broadside direction.

The catastrophic reflection of power, whether caused by end-fire gratinglobes or surface waves is a limitation on the maximum scan angle of thearray and accordingly a serious limitation on the performance of thearray.

It is, therefore, an object of the present invention to provide new andimproved phased array antennas which substantially reduce or eliminatethe problems of end-fire grating lobes and surface waves.

It is a further object of this invention to provide new and improvedphased array antennas which include a reactive impedance in thetransmission line formed by the ground plane and free space in order toeliminate the drastic effect produced by surface waves and end-firegrating lobes.

In accordance with the present invention, there is provided in a phasedarray antenna wherein at certain scan angles there may be produced aspurious TM wave along the ground plane which can cause substantiallyall the power to be undesirably reflected by the array resulting in adrastic reduction of radiated power, the combination comprising: aplurality of radiating elements for cooperatively producing a beam ofelectromagnetic energy which is capable of being steered in space by thecoupling of signals of differing phase to the radiating elements; aground plane associated with the plurality of radiating elements forpreventing substantial radiation behind the array; and means forproviding a reactive impedance in the transmision path formed by theground plane and free space for increasing the phase velocity of a TMwave which tends to propagate along the ground plane to a phase velocityat least equal to the phase velocity of a free space wave; whereby therange of scan angles over which there is no substantial reflection ofpower resulting from propagation along the ground plane is increased.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of thepresent invention together with other and further objects thereof,reference is had to the following description taken in conjunction withthe accompanying drawings, and its scope will be pointed out in theappended claims:

FIGS. la and lb illustrate one embodiment of the present invention;

FIG. 2 is a graphical representation of the radiation characteristics ofa typical phased array;

FIGS. 3a and 3!; illustrate the radiation characteristics of a singleelement over a ground plane;

FIG. 4a illustrates the equivalent circuit of a transmission line formedby the ground plane of a phased array and free space for a wavetraveling along the array surface;

FIG. 4b illustrates the equivalent circuit of such a transmission linehaving a series capacitance;

FIG. 5 illustrates the effect of a series capacitance on the equivalenttransmission line; FIGS. 60 and 6b illustrate another embodiment of thepresent invention;

FIGS. 7a and 7b illustrate another embodiment of the present invention;

FIGS. 8a and 8b illustrate another embodiment of the present invention;

FIGS. 9a and 9b illustrate a further embodiment of the present inventionwhich introduces shunt inductance into the transmission path formed bythe ground plane and free space, and

FIG. 10 is another embodiment of the present invention which introducesshunt inductance into the transmission path formed by the ground planeand free space.

DESCRIPTION OF THE INVENTION FIGS. la and lb illustrate one embodimentof the phased array antenna constructed in accordance with the presentinvention. FIG. la is a partial sectional view looking in at an edge ofthe antenna. FIG. lb is a partial front view of the antenna. The antennamay be considered to be an infinite array extending in all directions.The only difference between such an array and a practical array are theboundary conditions which are not relevant to the present invention.

The FIG. I antenna includes a plurality of radiating elementsillustrated as loop radiating elements 10 for cooperatively producing abeam of electromagnetic energy which is capable of being steered inspace by the coupling of the signals of different phase from signalgenerator 11 to each of the radiating elements 10. The manner in whichsignals of variable phase, coupled from signal generator ll, cooperateto produce a narrow steerable beam is well known in the art.

The antenna also includes a ground plane including a planar conductivesurface illustrated as conductive sheet 12 associated with the pluralityof radiating elements 10 for preventing substantial radiation behind thearray. Beside providing environmental protection, the ground planedirects the power from what would be an omnidirectional radiatingelement in the absence of the ground plane so that the element onlyradiates in front of the array thereby at least doubling the amount ofavailable power in the hemisphere in front of the array. In thoseconfigurations where the gound plane is a simple flush configuration,such as where the radiating elements consist of holes or slots in agound plane, the conductive sheet 12 is the ground plane. However. inthose configurations where the ground plane departs from the simpleflush configuration. such as when the metal sheet is covered by adielectric layer, the ground plane is a complex structure and theconductive sheet 12. although it is an esential element, does notconstitute the complete ground plane structure. The impedance of theground plane is affected by the structure which causes it to depart fromthe simple flush configuration and such structure is therefore a part ofthe ground plane.

The antenna further includes means for providing a reactive impedance inthe transmission path formed by the ground plane and free space forincreasing the phase velocity of a TM wave which tends to propagatealong the ground plane to a phase velocity which is at least equal tothe phase velocity of a free space wave. This impedance is provided bydielectric slab 13 which is coextensive with conductive sheet 12, andthc plurality of conductive pins 14, which are inserted in holes whichare regularly spaced along the surface of the dielectric slab l3,perpendicular to conductive sheet 12. Pins I4 are in a physical contactwith conductive sheet 12. The combination of conductive sheet 12,dielectric slab 13 and conductive pins 14 constitute a ground planehaving a series capacitance in the transmission path formed by theground plane and free space which increases the phmc velocity of a TMwave which tends to propagate along the ground plane so that it is atleast equal to the phase velocity of a free space wave without reducingthe phase velocity of a TE wave which tends to propagate along theground plane to less than that of a free space wave. This capacitiveground plane increases the range of scan angles over which there is nosubstantial reflection of power resulting from propagation along theground plane.

THEORY AND OPERATION OF THE INVENTION A typical prior art phased arrayantenna consists of a plurality of slots or holes in a metal groundplane. As previously explained gating lobes customarily accompany theradiation of the main beam and when the grating lobe is in the endfire"direction. there is a substantial reduction of radiated power. Thisphenomenon is illustrated in FIG. 2 which illustrates the typicalrelationship between the transmission coefficient plotted along theordinant with respect to the scan angle plotted along the abscissa.While FIG. 2 is based on values calculated for an infinite phased arrayit is equally applicable to a finite array having a large number ofradiating elements. Curve A of FIG. 2 represents the radiationcharacteristics of a simple flush mounted array and illustrates thecatastrophic reduction in radiated power that occurs when the gatinglobe is in the end-fire direction at scan angle 0,. There is almostcomplete reflection of the power at that scan angle.

Curve 8 in FIG. 2 illustrates the effects of surface wave propagationcaused by placing a dielectric sheet over the gound plane. At a scanangle which is less than the end-fire grating lobe angle there is acomplete reflection of energy as indicated by the absence of radiationat that angle. It can be seen that either the propagation of surfacewaves or the endfire gating lobe limits the range of scan angles overwhich the antenna can be utilized with the effect of surface waves beingpotentially even more catastrophic than end-fire gating lobes.

'lhe drastic reduction in radiated power caused by either end-firegating lobes or surface waves is a phenomenon associated withpropagation along the E plane, that is to say the condition will onlyexist when the radiated beam has acomponentlyingirrtheEplaneofthearrayandcurvesAandBof FIG. 2 arerepresentative of scanning in the E plane. Scanning in the H plane doesnot ordinarily produce the catastrophic reduction of power due to eitherend-fire grating lobes or surface waves as is illustrated by curve C.

The reason why the end-fire grating lobe does not ordinarily produce thecatastrophic effect of total reflection in the H plane is illustrated byFIGS. 30 and 3b which are a representation of the H plane and E planeradiation patterns, respectively, of a single element over a groundplane. As is shown in FIG. 30, there is no radiation along the groundplane for H plane scan while the element will radiate very stronglyalong the ground plane in the E plane. As a result the element does notcouple to an end-fire grating lobe in the H plane but strongly couplesto said end-fire grating lobe in the E plane.

Further where the ground plane departs from the simple flushconfiguration, the structure is also susceptible to the propagation ofsurface waves. Typically, prior art departures from a flushconfiguration resulted in the propagation of TM surface waves whereasthe departure usually did not have sufficient effect in the H plane toproduce TE surface waves.

In the E plane of scan, a wave propagating along the gound plane isexcited only in the TM mode. TM wave propagation along the ground planetherefore corresponds to E plane propagation. Similarly, in the H planeof scan, a wave propagating along the ground plane is excited only inthe TE mode. TE wave propagation along the ground plane thereforecorresponds to H plane propagation. Therefore as illustrate by curve Cin FIG. 2 there are no catastrophic reductions in radiated power in theH plane.

The FIG. I embodiment of this invention substantially reduces thedrastic effect produced by end-fire grating lobes and surface waves byintroducing a series capacitance into the transmission path formed bythe ground plane and free space which increases the phase velocity ofthe TM wave which tends to propagate along the gound plane to a phasevelocity at least equal to the phase velocity of a free space wave.

FIG. 4a illustrates the equivalent circuit of a transmission line formedby the ground plane and free space for a wave traveling along the arraysurface for an ordinary metal ground plane and is shown to consist of aseries inductor L land shunt capacitor C. FIG. 4b illustrates theaddition of a series capacitor C into this transmission line. the effectthereof being to increase the phase velocity of a wave which would tendto propagate along this transmission path. The increased wave velocityalong the ground plane may be thought as causing a refracting effectwhich bends the wave away from the ground plane as illustrated in FIG.5a. This results in a progressively weaker wave amplitude at the groundplane as the wave travels away from the radiating element. At a largedistance from the element there is little or no radiation near theground plane for any plane of polarization. The resulting radiationpattern for the slot is as illustrated in FIG. 5b which represents the Eplane radiation pattern of a single element over a ground plane whereinseries capacitance has been added to the transmission path formed by thegound plane and free space It will be noted that the radiation patternin the E plane now is similar to the radiation pattern in the H planeand accordingly the element will not radiate along the ground plane ineither plane of scan. in order to obtain the E plane as illustrated inFIG. 5b, sufficient series capacitance must be added so as to increasethe phase velocity of a wave traveling in the E plane so that it isgreater than the wave velocity of a free space wave.

Besides the elimination of the drastic effect of the end-fire gratinglobe, increasing the phase velocity along the ground plane in the Eplane also tends to eliminate the problems associated with thepropagation of surface waves. Increasing the phase velocity along theground plane in the E plane of scan so that it is just equal to thevelocity of a free space wave will provide the substantial benefit thatthe angle at which total reflection due to surface wave propagationoccurs. 6,, is increased to the angle at which the end fire grating lobeoccurs, 9 thereby increaing the usable range of scan angles. Furtherincreasing the phase velocity so that it is greater than the velocity ofa free space wave completely eliminates the catastrophic effect of totalreflection caused by either the endfire gating lobe or surface wavepropagation.

Although the addition of a series capacitance to the transmission pathformed by the ground plane and free space may eliminate the catastrophiceffects due to end-fire grating lobes or TM surface waves in the E planeof scan it is possible that the addition of series capacitance to thetransmission path 5 may cause effects in or near the H plane of scansimilar to those eliminated in the E plane. For waves traveling in the Hplane the series capacitance produced in a practical structure is likelyto vary with scan angle in such a way as to become an open circuit atsome angle thereby preventing radiation from the array at that angle.Furthermore, in a practical structure the dielectric or other loading islikely to have an effective thickness sufficiently great that a TEsurface wave may exist in or near the H plane. Such a surface wave wouldcreate the same kind of drastic reduction of radiated power that theintroduction of the series capacitance eliminated in the E plane of scanby preventing the propagation of TM surface waves.

The FIG. 1 embodiment provides the reactive impedance in the form of aseries capacitance in the transmission path formed by the ground planeand free space that increases the phase velocity of a TM wave tending topropagate along the ground plane so that it is at least equal to thevelocity of a free space wave without reducing equal to the velocity ofa TM wave which tends to propagate along the ground plane to less thanthat of a free space wave thereby increasing the range of usable scanangles in the E plane without undesirably effecting the range of usablescan angles in the H plane. A TM wave incident on the face of thedielectric at any angle is constrained by pins 14 and made to travel inthe direction perpendicular to conductive sheet 12. Pins 14 in effectprovide a plurality of transmission lines which are short circuited byconductive sheet 12. As is well known a shorted transmission lineprovides a reactive impedance. At a distance between one-quarter andone-half wavelength from the shon circuit, the impedance is capacitive.Therefore, if the thickness of dielectric slab l3 and accordingly thelength of pins 14 is between one-quarter and one-half wavelength thefront surface 13' of dielectric slab 13 will present a capacitiveimpedance to a TM wave.

A TE wave incident on the face of the array has an electric field vectorperpendicular to pins 14. If the diameter of pins 14 is small enough,they appear to be invisible to an electrical field vector orientedperpendicular to them. TE pins 14 have no substantial effect on thepropagation of a the wave.

The dielectric constant K of the slab and thickness of the slab andaccordingly the length of the pins L are determined by first determiningthe capacitive reactance X,., relative to a free space impedance Z thatis required. As previously stated the amount of capacitance is relatedto the amount of wave speeding required. The phase velocity of the wavemust be increased so that it is at least equal to the velocity of a freespace wave and preferably so that it is greater than a free space wave.The amount of series capacitance to be introduced by the combination ofdielectric slab 13 and metal pins 14 is accordingly determined by thereactive impedance of the transmision path that would exist absent thiscombination.

Having determined=the amount of capacitive reactance, X required, thevalues of L and K are determined by solving the following transcendentalequation for gl mined and the freespace wavelength )t The actual valuesfor K and L should be slightly less than those derived in accordancewith the above equation in order to guarantee that a the surface wave isnot created by the structure for waves in or near the H plane. Therewill still be an angle of radiation in the H plane where the structureplaces an open circuit in series with the equivalent transmission line,thereby preventing radiation. However, by using the values describedabove, this angle of nonradiation will be very close to the directionparallel to the array face, the end-fire direction, and planar phasedarrays are not ordinarily operated near the end-fire direction.

The principle by which the FIG. 1 embodiment avoids the harmful effectspossible for waves radiated in or near the H plane is that the structureappears to be an open circuit to an incident wave traveling in the Hplane at an incidence angle very close to grazing incidence, theend-fire direction. For a certain value of K there is only one value ofL which achieves this and vice versa as determined by equation (2). If Kor L is too large, a the surface wave in and near the H plane can exist.If K or L is too small the array impedance will become an open circuitfor H plane radiation at an angle not very close to grazing, andtherefore closer to the operating scan angles of the array.

Metal pins 14 maintain the capacitive reactance of the ground planesubstantially constant for all angles of radiation in the E plane. Sincean incident wave in the E plane is guided by pins 14 into a path nearlyperpendicular to the face of the structure, substantially constant pathlength is maintained for an incidence angle. As previously stated, thispath length in dielectric material 13 is some value between a quarterwavelength and half wavelength depending on the desired capacitivereactance X,.

Although the FIG. 1 embodiment provides this series capacitance in thetransmission path formed by the ground plane and free space so a toeliminate the end-fire grating lobe and surface wave problems in the Eplane without affecting the H plane, the resultant antenna has aninherent limitation in that wire antennas tend to become impractical forradiating energy having a wavelength less than an inch. FIGS. 6a and 6!:illustrate a portion of a phased array antenna constructed insubstantially the same manner as the FIG. I antenna but having narrowdiameter waveguides as the radiating elements.

As in FIG. 1, selected ones of the holes in dielectric slab 13,positioned at regularly spaced intervals, have a larger diameter thanthe remainder of the holes. Within each of the larger holes ispositioned a hollow conductive cylinder 15 which is in contact withconductive sheet 12. Although having a larger diameter than the diameterof pins 14 each cylinder 15 has an internal diameter less than thatrequired to propagate the energy diameter from source 11 as an unloadedwaveguide. However, as is well known a narrow diameter waveguide can becaused to propagate by "loading" the waveguide. Internally modifying thewaveguide by filling the inside with high-k dielectric material 16 asillustrated in FIG. 6 or placing metal ridges inside the waveguideparallel to the electric field vector lowers the cutoff frequency of thewaveguide so that it will propagate the lowest mode. This configurationprovides a wave speeding effect for a TM wave without any deleteriouseffect on a TE wave.

The radiating elements in the FIGS. 1 and 6 embodiments have a narrowbandwidth. FIG. 7 illustrates another embodiment of a phased arrayantenna constructed in accordance with the present invention having abroader bandwidth. FIG. 7a is a partial sectional view looking in at theedge of the antenna while FIG. 7b is a front view of FIG. 7a. Theantenna includes a plurality of radiating elements illustrated aswaveguides 17 for producing a steerable beam as a result of the couplingof signals of variable phase from signal generator 11. The antenna alsoincludes a ground plane including conductive sheet 12 associated withthe plurality of radiating elements 17 for preventing substantialradiation behind the array.

The FIG. 7 antenna provides series capacitance in the transmission pathformed by the ground plane and free space in substantially the samemanner as the FIG. I and FIG. 6 cmbodiments with the principaldifference that there are no pins between the radiating elements 17. InFIG. 7 the radiating elements, waveguides 17, provide an action similarto that of pins 14 in FIGS. 1 and 6. In this embodiment the waveguideshave a larger diameter than the wageguides of FIG. 6 thereby permittinga better impedance match to frequency bandwidth. However, the largerwaveguides have a loading effect on the ground plane similar to thatofan artificial dielectric. Therefore, the K of the real dielectric 13between waveguides I7 must be reduced with respect to the valuecalculated in conjunction with the description of FIG. 1 in order tomaintain the condition ofan open circuit to an incident wave travelingin the H plane at an incidence angle very close to grazing incidence.

FIG. 8 shows a still further embodiment of the present invention whichprovides a series capacitance in the transmis sion path formed by theground plane and free space for increasing the phase velocity of a TMwave which tends to propagate along the ground plane so that it is atleast equal to the phase velocity of a free space wave without reducingthe phase velocity ofa TE wave to less than the velocity of a free spacewave. In FIG. 8 the diameter of waveguides 18 has been enlarged withrespect to the FIG. 8 embodiment so that their artificial dielectricloading effect provides the desired open circuit condition without anyreal dielectric material between waveguides l8.

Waveguides l8 serve a function analogous to that of pins 14 in the FIG.1 embodiment, namely, that an incident TM wave is constrained bywaveguides l8 and made to travel in the direction perpendicular toconductive sheet I2.

lfa series capacitance is to be presented to an incident TM wave as aresult of a constraining effect produced by waveguides 18, the length Lof each waveguide 18 above conductive sheet 12 must be between M4 and M2where A is the freespace wavelength of the radiated energy. However, aswill be more fully explained, flanges l9 permit the length L to be lowthan H4 and still introduce a series capacitance in the transmissionpath fonned by the ground plane and free space.

As indicated, each of waveguides 18 in FIG. 8 include a metal flange 19.As is more fully illustrated in FIG. 8b, flanges 19 are constructed sothat a space exists between flanges 19 associated with adjacentwaveguides. Any adjacent pair of flanges 19 therefore comprises a pairof conductors separated by a dielectric (air) and it is thereforeapparent that flanges 19 provide a lumped capacitance to a TM waveincident on the ground plane.

Since flanges 19 provide a lumped capacitance which is in parallel withthe reactive impedance produced by the constraining cffect provided bywaveguides 18 the length L' of waveguides 18 may be somewhat less thanM4. Any inductive reactance provided by the constraining effect ofwaveguides 18 would be overcome by the capacitance provided by flanges19 to provide a resultant series capacitance in the transmission pathformed by the ground plane and free space.

FIG. 9 is another embodiment of a phased array antenna constructed inaccordance with the present invention which includes a plurality ofradiating elements associated with a conductive sheet 12, each radiatingelement includes a waveguide ending in a hole in the conductive sheet12. As previously explained, conductive sheet 12, absent any structurewhich disturbs the flush configuration, would constitute the groundplane. However, disturbance of the ground plane by the addition of thestructure described below makes the ground plane a complex structure ofwhich conductive sheet 12 is an essential part.

The antenna further includes a plurality of thin conductive wires 20each attached at one end to conductive sheet 12 and arrangedperpendicular to it at regular intervals for providing a shuntinductance in the transmission path formed by the ground plane and freespace for increasing the phase velocity of a TM wave which tends topropagate along the ground plane to a phase velocity at least equal tothe phase velocity of a free space wave without reducing the phasevelocity of a TE wave which tends to propagate along the ground plane toless than that of a free space wave. As previously explained, increasingthe phase velocity of a TM wave that tends to travel along the groundplane provides improved performance by reducing or eliminating theeffect of grating lobes and surface waves. For the same reason that aseries capacitance achieves this result the shunt inductance introducedby wires 20 provides the desired increase in phase velocity.

Since wires 20 are located across the transmission path formed by theground plane and free space and are grounded by conductive sheet 12 theyprovide a shunt reactance to a TM wave traveling in that transmissionpath. If the length of wires 20 in front of conductive sheet 12 isbetween M4 and M2, the shunt reactance provided is inductive. Thedimension 2] is therefore between M4 and M2. In order to provide thedesired inductance the wires must be very thin with respect to awavelength; i.e., in the order to 0.001). The spacing between wires 20is dependent on the amount of inductance required.

Since the electric field vector of a TE wave traveling in thetransmission path formed by the ground plane and free space isperpendicular to wires 20, they appear to be invisible to that vectorand have no substantial effect on the corresponding TE wave.

It should be noted that the thin conductive wires 20, although inappearance similar in structure to pins 14 of the FIG. 1 embodiment,provide a shunt inductance since they are in front of the radiatingelements and therefore in the trans mission path formed by the groundplane and free space while the pins 14 are behind the radiatingelements.

FIG. 10 is another embodiment of the present invention which providesshunt inductance in the transmission path formed by the ground plane andfree space in substantially the same manner as the FIG. 9 embodiment.The FIG. 10 embodiment consists of thin inductive wires 22 which areencapsulated in the dielectric material 23 and top loaded by the metaldiscs 24. Metal discs 24 collectively provide wide angle impedancematching as is fully described in the copending application of Peter W.Harman, Ser. No. 815,566, filed Apr. l4, I969 and entitled PHASED ARRAYANTENNA INCLUD- ING IMPEDANCE MATCHING APPARATUS." Whereas in the abovereferenced application the metal discs were mounted on thin dielectricsupporting discs in order to avoid the propagation of surface waves,discs 24 are mounted on solid cylinders of dielectric material 23 whichcontribute to the impedance matching efiect provided by discs 21. Solidcylinders of dielectric material 20 can be utilized since the thinconductive wires 22 increase the phase velocity so as to avoid surfacewave propagation as described above.

In the present embodiment, besides providing the desired impedancematching, metal discs 24 serve to top load thin wires 22. Wires 22 cantherefore have an actual length less than a quarter wavelength whilehaving an eflective length between H4 and M2 thereby presenting a shuntinductance to a TM wave.

Although the inductive wires 22 in FIG. 10 are not directly connected toconductive sheet 12, as in FIG. 9, the insertion of the wire into theradiating element by approximately M4 makes it appear that the wire isconnected to conductive sheet 12. The portion ofwire 22 in thewaveguide, which ends at opening 10, forms a transmission path which isopen circuited at end 25 of wire 23. A point a quarter wavelength froman open circuit in a transmission line appears to be a short circuit.Therefore, if point 26, being the point where the plane of conductivesheet 12 intersects wire 22, is a quarter wavelength from point 24, wire22 will appear to be shorted to conductive sheet 12 or at least have alow impedance connection thereto, at that point. Wires 22 thereforeprovide a shunt inductance in substantially the same manner as wires 18in FIG. 9.

While there have been described what are at present considered to be thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention and it is, therefore, aimedto cover all such changes and modifications as fall within the truespirit and scope of the invention.

l claim:

1. In a phased array antenna wherein at certain scan angles there may beproduced a spurious TM wave along the ground plane which can causesubstantially all the power to be undesirably reflected by the arrayresulting in a drastic reduction of radiated power, the combinationcomprising:

a plurality of radiating elements for cooperatively producing a beam ofelectromagnetic energy which is capable of being steered in space by thecoupling of signals of differing phase to the radiating elements;

a ground plane associated with said plurality of radiating elements forpreventing substantial radiation behind the array;

and means for providing a reactive impedance in the transmission pathformed by said ground plane and free space for increasing the phasevelocity of a TM wave which tends to propagate along the ground plane toa phase velocity at least equal to the phase velocity of a free spacewave;

whereby the range of scan angles over which there is no substantialreflection of power resulting from propagation along the ground plane isincreased.

2. in a phased array antenna wherein at certain scan angles there may beproduced a spurious TM wave along the ground plane which can causesubstantially all the energy to be undesirably refiected by the arrayresulting in a drastic reduction of radiated power, the combinationcomprising:

a plurality of radiating elements for cooperatively producing a beam ofelectromagnetic energy which is capable of being steered in space by thecoupling of signals of differing phase to the radiating elements;

a ground plane including a planar conductive surface as sociated withsaid plurality of radiating elements for preventing substantialradiation behind the array;

and means for providing a series capacitance in the transmission pathfomied by said ground plane and free space for increasing the phasevelocity of a TM wave which tends to propagate along the ground plane toa phase velocity at least equal to the phase velocity of a free spacewave without reducing the phase velocity of a TE wave which tends topropagate along the ground plane to less than that of a free space wave;

whereby the range of scan angles over which there is no substantialreflection of power resulting from propagation along the ground plane isincreased.

3. In a phased array antenna the combination as claimed in claim 2 inwhich the means for providing a series capacitance includes a dielectricslab coextensive with said planar conductive surface having a pluralityof regularly spaced holes perpendicular to said conductive surface and aplurality of thin conductive pins each positioned in one of said holesand in contact with said conductive surface at one end thereof, thethickness of said slab and the length of said pins being greater than).,,/4 and les than AJZ, where A, is the wavelength of saidelectromagnetic energy in said dielectric.

4. in a phased array antenna the combination as claimed in claim 3 inwhich selected ones of said holes positioned at regularly spacedintervals have a larger diameter than the remainder of said holes andwhich additionally includes a plurality of hollow conductive cylinderseach positioned in one of said selected holes and in contact with saidconductive surface at one end thereof and in which said radiatingelements consists of thin wire loops positioned in front of saiddielectric slab and connected to a signal source through said hollowconductive cylinders.

5. In a phased array antenna the combination as claimed in claim 3 inwhich selected ones of said holes positioned at regularly spacedintervals have a larger diameter than the remainder of said holes andwhich additionally includes a plurality of hollow conductive cylinderseach positioned in one of said selected holes in contact with saidconductive surface at one end thereof, each cylinder having an internaldiameter less than the diameter required to propagate saidelectromagnetic energy as an unloaded waveguide and in which eachradiating element consists of a loaded waveguide including saidconductive cylinders internally modified to propagate saidelectromagnetic energy.

6. In a phased array antenna the combination as claimed in claim 2 whichincludes a plurality of loaded metal wall waveguides comprising saidplurality of radiating elements, each of said waveguides protruding fromsaid planar conductive surface, the length of each waveguide above saidconductive surface being between M4 and M2 where )t is the free spacewavelength of the radiated energy.

7. in a phased array antenna the combination as claimed in claim 2 whichincludes a plurality of loaded metal wall waveguides protruding fromsaid planar conductive surface comprising said plurality of radiatingelements, each of said waveguides having a flange positioned betweensaid conductive surface and free space, flanges of adjacent waveguidesbeing in spatial relationship so as to present a lumped capacitance toan incident TM wave.

8. In a phased array antenna wherein at certain scan angles there may beproduced a spun'ous TM wave along the ground plane which can causesubstantially all the energy to be undesirably reflected by the arrayresulting in a drastic reduction of radiated power, the combinationcomprising:

a plurality of radiating elements for cooperatively producing a beam ofelectromagnetic energy which is capable of being steered in space by thecoupling of signals of differing phase to the radiating elements;

a ground plane including a planar conductive surface associated withsaid plurality of radiating elements for preventing substantialradiation behind the array;

and means for providing a shunt inductance in the transmission pathformed by said ground plane and free space for increasing the phasevelocity of a TM wave which tends to propagate along the ground plane toa phase velocity at least equal to the phase velocity of a freespacewave without reducing the phase velocity of a TE wave which tends topropagate along the ground plane to less than that of a freespace wave;

whereby the range of scan angles over which there is no substantialreflection of power resulting from propagation along the ground plane isincreased.

9. In a phased array antenna the combination as claimed in claim 8 inwhich the means for providing a shunt inductance includes a plurality ofthin conductive wires oriented in the transmission path formed by saidground plane and free space substantially perpendicular to said planarconductive surface, connected at one end thereto and having an effectivelength between M4 and All, where A is the freespace wavelength of saidelectromagnetic energy.

10. [n a phased array antenna the combination as claimed in claim 9which additionally includes dielectric material encapsulating andsupporting said thin conductive wires.

11. In a phased array antenna the combination as claimed in claim 8 inwhich the means for providing a shunt inductance includes a plurality ofthin conductive wires oriented in the transmission path formed by saidground plane and free space substantially perpendicular to said planarconductive surface, said radiating elements include metal wallwaveguides terminating in openings in said conductive surface and whichadditionally includes a plurality of solid cylinders of dielectricmaterial each positioned within one of said waveguides and having a lowimpedance connection at one end to said thin metal discs and at theopposite end to said conductive surface 13. In a phased array antennathe combination as claimed in claim 12 in which said thin wires projectinto said waveguides by a distance equal to M4.

1. In a phased array antenna wherein at certain scan angles there may beproduced a spurious TM wave along the ground plane which can causesubstantially all the power to be undesirably reflected by the arrayresulting in a drastic reduction of radiated power, the combinationcomprising: a plurality of radiating elements for cooperativelyproducing a beam of electromagnetic energy which is capable of beingsteered in space by the coupling of signals of differing phase to theradiating elements; a ground plane associated with said plurality ofradiating elements for preventing substantial radiation behind thearray; and means for providing a reactive impedance in the transmissionpath formed by said ground plane and free space for increasing the phasevelocity of a TM wave which tends to propagate along the ground plane toa phase velocity at least equal to the phase velocity of a free spacewave; whereby the range of scan angles over which there is nosubstantial reflection of power resulting from propagation along theground plane is increased.
 2. In a phased array antenna wherein atcertain scan angles there may be produced a spurious TM wave along theground plane which can cause substantially all the energy to beundesirably reflected by the array resulting in a drastic reduction ofradiated power, the combination comprising: a plurality of radiatingelements for cooperatively producing a beam of electromagnetic energywhich is capable of being steered in space by the coupling of signals ofdiffering phase to the radiating elements; a ground plane including aplanar conductive surface associated with said plurality of radiatingelements for preventing substantial radiation behind the array; andmeans for providing a series capacitance in the transmission path formedby said ground plane and free space for increasing the phase velocity ofa TM wave which tends to propagate along the ground plane to a phasevelocity at least equal to the phase velocity of a free space wavewithout reducing the phase velocity of a TE wave which tends topropagate along the ground plane to less than that of a free space wave;whereby the range of scan angles over which there is no substantialreflection of power resulting from propagation along the ground plane isincreased.
 3. In a phased array antenna the combination as claimed inclaim 2 in which the means for providing a series capacitance includes adielectric slab coextensive with said planar conductive surface having aplurality of regularly spaced holes perpendicular to said conductivesurface and a plurality of thin conductive pins each positioned in oneof said holes and in contact with said conductive surface at one endthereof, the thickness of said slab and the length of said pins beinggreater than lambda d/4 and less than lambda d/2, where lambda d is thewavelength of said electromagnetic energy in said dielectric.
 4. In aphased array antenna the combination as claimed in claim 3 in whichselected ones of said holes positioned at regularly spaced intervalshave a larger diameter than the remainder of said holes and whichadditionally includes a plurality of hollow conductive cylinders eachpositioned in one of said selected holes and in contact with saidconductive surface at one end thereof and in which said radiatingelements consists of thin wire loops positioned in front of saiddielectric slab and connected to a signal source through said hollowconductive cylinders.
 5. In a phased array antenna the combination asclaimed in claim 3 in which selected ones of said holes positioned atregularly spaced intervals have a larger diameter than the remainder ofsaid holes and which additionally includes a plurality of hollowconductive cylinders each positioned in one of said selected holes incontact with said conductive surface at one end thereof, each cylinderhaving an internal diameter less than the diameter required to propagatesaid electromagnetic energy as an unloaded waveguide and in which eachradiating element consists of a loaded waveguide including saidconductive cylinders internally modified to propagate saidelectromagnetic energy.
 6. In a phased array antenna the combination asclaimed in claim 2 which includes a plurality of loaded metal wallwaveguides comprising said plurality of radiating elements, each of saidwaveguides protruding from said planar conductive surface, the length ofeach waveguide above said conductive surface being between lambda /4 andlambda /2 where lambda is the free space wavelength of the radiatedenergy.
 7. In a phased array antenna the combination as claimed in claim2 which includes a plurality of loaded metal wall waveguides protrudingfrom said planar conductive surface comprising said plurality ofradiating elements, each of said waveguides having a flange positionedbetween said conductive surface and free space, flanges of adjacentwaveguides being in spatial relationship so as to present a lumpedcapacitance to an incident TM wave.
 8. In a phased array antenna whereinat certain scan angles there may be produced a spurious TM wave alongthe ground plane which can cause substantially all the energy to beundesirably reflected by the array resulting in a drastic reduction ofradiated power, the combination comprising: a pluRality of radiatingelements for cooperatively producing a beam of electromagnetic energywhich is capable of being steered in space by the coupling of signals ofdiffering phase to the radiating elements; a ground plane including aplanar conductive surface associated with said plurality of radiatingelements for preventing substantial radiation behind the array; andmeans for providing a shunt inductance in the transmission path formedby said ground plane and free space for increasing the phase velocity ofa TM wave which tends to propagate along the ground plane to a phasevelocity at least equal to the phase velocity of a freespace wavewithout reducing the phase velocity of a TE wave which tends topropagate along the ground plane to less than that of a freespace wave;whereby the range of scan angles over which there is no substantialreflection of power resulting from propagation along the ground plane isincreased.
 9. In a phased array antenna the combination as claimed inclaim 8 in which the means for providing a shunt inductance includes aplurality of thin conductive wires oriented in the transmission pathformed by said ground plane and free space substantially perpendicularto said planar conductive surface, connected at one end thereto andhaving an effective length between lambda /4 and lambda /2, where lambdais the freespace wavelength of said electromagnetic energy.
 10. In aphased array antenna the combination as claimed in claim 9 whichadditionally includes dielectric material encapsulating and supportingsaid thin conductive wires.
 11. In a phased array antenna thecombination as claimed in claim 8 in which the means for providing ashunt inductance includes a plurality of thin conductive wires orientedin the transmission path formed by said ground plane and free spacesubstantially perpendicular to said planar conductive surface, saidradiating elements include metal wall waveguides terminating in openingsin said conductive surface and which additionally includes a pluralityof solid cylinders of dielectric material each positioned within one ofsaid waveguides and protruding through said openings into free space,said cylinders encapsulating said thin conductive wires.
 12. In a phasedarray antenna the combination as claimed in claim 11 which additionallyincludes a plurality of thin metal discs, each positioned in one of saidcylinders on the surface parallel to said conductive surface said thinconductive wires having a low impedance connection at one end to saidthin metal discs and at the opposite end to said conductive surface. 13.In a phased array antenna the combination as claimed in claim 12 inwhich said thin wires project into said waveguides by a distance equalto lambda /4.